Since nanofibers are generally ultrafine fibers having a diameter (a width) of several tens to several hundreds of nanometers with a feature of having noticeably larger surface area than that of a conventional fiber, nanofibers are focus of attention as those which are novel and exert a special function, and thus utilization thereof has made progress. Polymer materials such as nylon and polyester are mainly used as raw materials of nanofibers. Recently, it has also intensively studied that nanofibers are obtained from biological materials and are utilized from the viewpoint of environmental concerns.
Chitin and chitosan are also derived from organisms, and a study on conversion into nanofibers has been made. For example, there are methods in which a commercially available chitin is converted into nanofibers by fibrillizing and drying using a rotary wet disc grinder (Patent Document 1, Patent Document 2). However, since the commercially available chitin forms a hydrogen bond between fibers, very strongly, it is difficult to completely fibrillized fibers even when physical loads are applied, and thus each fiber has an irregular shape. There are also examples in which chitosan is dissolved in a solvent and nanofibers are spun by electrospinning (Patent Document 3, Non-Patent Document 1). However, it is necessary to dissolve chitosan once in the solvent, and it is not suited for mass production because of huge environmental burdens. Fibers obtained by electrospinning have a large diameter (fiber width of 100 nm or more) and are not uniform. It is difficult to carry out mass production by an electrospinning method, and energy costs are high. Since chitin is insoluble in the solvent, chitin nanofibers cannot be produced by the electrospinning method. There is also a method of obtaining bionanofibers by hydrolysis (Non-Patent Document 2). However, since fibers are cut into pieces by an acid treatment, the fiber length decreases to 1 μm or less. There is also a method in which a commercially available chitin is subjected to an oxidation treatment using a TEMPO catalyst thereby enhancing dispersibility in water, and then nanofibers are fibrillized by an ultrasonic treatment (Non-Patent Document 3). However, in this method, chitin is hydrolyzed by the oxidation treatment and the fiber length largely decreases. The obtained product is akin to a whisker rather than a fiber and also the oxidation treatment is carried out, and thus the product has a chemical structure which is strictly different from that of chitin. There are also methods in which acetic acid is added to chitin derived from squid tendons and the mixture is converted into nanofibers by an ultrasonic treatment (Patent Document 4, Non-Patent Document 4). Since chitin derived from squid is bata-chitin having low crystallinity and is likely to be fibrillized, it is possible to be converted into nanofibers. However, chitin derived from crab and prawn shells is alpha-chitin which has high crystallinity and strong mechanical strength, and thus chitin nanofibers cannot be obtained even when subjected to similar treatment. Furthermore, since squid tendons are overwhelmingly poor in quantity of resources when compared with crab and prawn shells, there is low possibility that the aforementioned method is put into practice.
Shellfishes (Crustacea) such as crab and prawn richly contain chitin in the integument. Moreover, crab and prawn are consumed in a large amount. In almost all cases, these integuments are discarded. Therefore, some trials have been made to obtain chitin from shellfishes thereby producing nanofibers in order to effectively utilize these resources. However, there have never been obtained chitin nanofibers, which are thin and long in a state as it is, and are also homogeneous and excellent in any of crystallinity, physical properties, simplicity of a treatment operation and an accumulation amount, from these organisms.
Uses of nanofibers derived from organism have also been studied and have been put into practice. For example, a coating material utilizing chitosan has been commercialized. However, since chitosan is only soluble in an acidic solution, the coating material utilizing chitosan is not suited for metal. Also, chitosan has high hygroscopicity, and thus the coating material utilizing chitosan must be painted during the summer season in the place where an air conditioner is present.
Patent Document 1: JP-A-2003-155349
Patent Document 2: JP-A-4-281017
Patent Document 3: JP-A-2007-236551
Patent Document 4: JP-A-2009-102782
Non-Patent Document 1: Min B. et al., Polymer, 2004, 45, 7137
Non-Patent Document 2: Gopalan N., et al., Biomacromolecules, 2003, 4, 657
Non-Patent Document 3: Fan Y. et al, Biomacromolecules, 2008, 9, 192.
Non-Patent Document 4: Fan Y. et al, Biomacromolecules, 2008, 9, 1919.